This invention relates generally to computed tomography and more particularly to cone beam volumetric computed tomography.
An interference caused by x-ray scatter can undermine CT image quality and the validity of CT-based diagnostic imaging. In a single-row fan beam (FB) CT, the x-ray scatter is substantially reduced or almost eliminated through (1) shutting the aperture of a x-ray source collimator to a slit as the severity of x-ray scatter is proportional to the area illuminated by an x-ray beam, and (2) utilization of a detector collimator, which is cylindrically symmetric and focus at the focal spot of the x-ray source, to shield scattered x-ray photons but pass primary x-ray ones as much as possible. In a contemporary multi-row (up to 16) FB CT, those two measures are still being employed successfully to combat x-ray scatter interference. Hereinafter, both the single-row FB CT and multi-row FB CT are called FB CT. However, with a significant increment in row number, e.g., above 64, of a x-ray detector in cone beam (CB) volumetric CT, the techniques mentioned above will not work as well as their counterparts in FB CT, because (a) the aperture of x-ray source collimator is typically opened relatively wide in CB volumetric CT, and (b) the grid ratio of cylindrically symmetric detector collimator in volumetric CB CT can no longer be as high as that of their counterparts in FB CT, since an x-ray beam is intrinsically in sphere symmetry, and the uniformity of dynamic quantum efficiency (DQE) of an x-ray detector will be compromised by a cylindrically symmetric detector collimator if the grid ratio is as high as those in FB CT.